The marine carbon dioxide (CO2) system plays a critical role in regulating CO2 fluxes into and out of the world's oceans. One of the primary mechanisms by which the ocean affects the Earth's climate is through regulating CO2 gas into and out of the ocean via the marine CO2 system. Currently, the ocean absorbs about one third of the anthropogenic CO2 released to the atmosphere by human activities, thus playing a major role in reducing the rate of atmospheric CO2 increase and thereby curbing global warming. However, oceanic uptake of anthropogenic carbon is causing a rapid change in seawater carbonate chemistry, often referred to as ocean acidification, wherein excess CO2 lowers seawater pH, increases total CO2 concentration, and decreases calcium carbonate saturation. Changes in the marine CO2 system may result in complicated responses and feedbacks in the ocean, ranging from changes in marine carbon and other elemental cycles to marine biology and ecology. Ocean acidification also reduces seawater buffering capacity, slowing down oceanic carbon uptake and acting as a positive feedback to the atmospheric CO2 increase
The four primary parameters used to characterize the marine CO2 system are total dissolved inorganic carbon (DIC), partial pressure of CO2 (pCO2) or CO2 fugacity (fCO2), pH, and total alkalinity (TA). DIC is defined as the sum of all carbonic acid species in water: DIC=CO2*+HCO3−+CO32−, where CO2* is the sum of dissolved CO2 and carbonic acid (H2CO3). DIC is a master carbon parameter frequently used to study, identify, and differentiate many processes linked to the marine carbon cycle (e.g. biological uptake of CO2, ocean acidification, and anthropogenic CO2 penetration in the ocean). The assessment of these processes ultimately relies on high-quality measurements of seawater DIC. In addition, to fully characterize the CO2 system through thermodynamic calculations, at least two CO2 parameters must be measured. CO2 calculations made using DIC data as one of the parameters yield results that are often more consistent with measured values. Because of its important role in the CO2 system, DIC was measured during all of the major ocean carbon expeditions, such as the Climate Variability and Predictability (CLIVAR) Hydrography Program and the Joint Global Ocean Flux Study (JGOFS).
Theoretically, measurements of any two of the four parameters along with salinity and temperature can be used to calculate the other parameters and fully resolve carbonate chemistry using seawater acid-base equilibria. However, selection of different measurement pairs in practice will generate a range of calculation errors resulting from analytical errors, uncertainties in equilibrium constants, and their non-linear propagation in calculation. Using DIC or TA as one of the measured pair produces relatively small calculation errors, while selection of the pCO2-pH pair for measurements causes large calculation bias even under the best analytical practice. Only in situ pCO2 and pH measurements have become increasingly common in recent years on various platforms, such as buoys and profilers, as commercial pCO2 and pH sensors are available. In contrast, in situ sensing for DIC and TA are much less mature, and are mostly under different development stages. Simultaneous, in situ measurements of two CO2 system parameters with either DIC or TA as one of the two are highly desirable but extremely rare.
Traditional bottle sampling and subsequent analysis of DIC can only achieve limited spatiotemporal coverage mainly because of associated high costs and low throughput. Development of methodologies that are suitable for high-resolution in situ measurements of CO2 parameters have been widely recognized as a research priority in the carbon and ocean acidification research community. Among various methods (e.g. coulometry, potentiometry, non-dispersive infrared (NDIR) method, and conductimetry) developed for high-precision DIC measurements, the spectrophotometric method offers high sensitivity, good stability, and direct measurements of water-phase samples. It can be ‘calibration-free’ in theory, thus reducing maintenance requirements. These attributes make it well suited for in situ underwater applications.
The existing spectrophotometric DIC method is based on spectrophotometric pH measurements where observed absorbances of a sulfonephthalein indicator liquid and its equilibrium properties are used to quantify sample pH. A piece of Teflon AF 2400 (DuPont™ copolymer) capillary tubing is used as both an optical cell and a CO2 equilibrator as it is highly permeable to CO2 molecules and can act as a liquid-core waveguide (LCW) for optical detection. The spectrophotometric detection occurs after full CO2 equilibration is established between the acidified sample and the indicator solution across the Teflon AF tubing. The indicator solution is motionless during the equilibration process. This method is similar in principle to the spectrophotometric fCO2 method, but the sample is not acidified and a different indicator is used. Because the indicator does not directly mix with the sample in either of these methods, no dilution or perturbation to the seawater sample occurs.
The response time (i.e. the time required to obtain a stable reading for detection) of the existing spectrophotometric method is about 5 minutes, which is the CO2 exchange time required to reach full CO2 equilibration. This method has been used for underway measurements of flow-through seawater, and actual measurements are intermittent. Such a response is sufficient for some stationary measurements, such as bottle samples and buoy deployments, where discontinuous measurements are acceptable. However, it is not ideal for high-resolution measurements made on mobile platforms, particularly those such as Automated Underwater Vehicles (AUVs), Remotely Operated Vehicles (ROVs), gliders, or water-column profilers. At the 5-minute sampling interval, the spatiotemporal resolution on these mobile platforms may be limited for studying rapid changes on a scale down to minutes or meters and fine-scale features such as those encountered in coastal oceans and water-column profiling.